They've had pretty bad episodes...
The first few seasons, for instance, were really nothing special at all... no better really than the first few seasons of family guy, and nowadays they really seem like they've been all over the place as far as their quality goes.
I completely disagree, I'd say parts of season 14 and the first half of season 15 are the weakest for sure.
I adore seasons 1 and 2- they're a little heavier in the fart/poop jokes department but the storylines are far more solid and comedy less in your face than the most recent seasons.
I think season 3 is possibly the best season of all, equal or perhaps slightly ahead of 6.
Eh, a bit late but whatever... the first 2 seasons mostly suffered from (everything trey mentioned in 6 days to air... less money, worse technology, and the fact the fact that it was mostly just them and Isaac then) They were still funny, I like most everything they've done really... I just think they improved DRASTICALLY midway down the road... I think 6 and 7 are wayyy better but that's just my opinion of course.